1. Field of the Invention
The present invention relates to an occupant restraint apparatus having an impact energy absorbing mechanism for restraining an occupant while preventing a seat belt from being paid out in an emergency and allowing the seat belt to be paid out when a load in excess of a preset load is applied to the occupant, and a method of controlling such an occupant restraint apparatus.
2. Description of the Related Art
Generally, occupant restraint apparatus such as seat belt apparatus are effective to protect vehicle occupants against injury upon collision of the vehicle. For example, a seat belt apparatus has an emergency locking retractor (hereinafter referred to as an "ELR") for winding back a seat belt known as a webbing around a take-up shaft under spring forces and preventing the seat belt from being paid out in order to restrain an occupant when subjected to a collision-induced impact.
When the seat belt is prevented from being paid out by the ELR, since the occupant is abruptly retrained against forward movement, the occupant suffers from an impact force imposed via the seat belt. In order to reduce such an impact force applied to the occupant, there has been employed an impact energy absorbing mechanism (hereinafter referred to as an "EA mechanism") for paying out the seat belt while keeping the seat belt under a predetermined webbing tension (hereinafter referred to as an "EA load") thereby to absorb impact energy acting on the occupant when a load in excess of a preset load acts on the seat belt after the seat belt is locked by the ELR, as disclosed in Japanese laid-open patent publication No. 8-127313, for example.
As indicated by an equation of motion: Fs (restraint force)=m (mass of the occupant).times..alpha. (acceleration or deceleration), if the mass of the occupant, i.e., the weight of the occupant, differs under a constant EA load, then the acceleration (deceleration or impact) applied to the occupant upon vehicle collision also differs as shown in FIG. 20 of the accompanying drawings. Japanese laid-open patent publication No. 8-268224 discloses a technique for varying the EA load by making mechanical adjustments to meet the weight of the occupant. The disclosed arrangement is disadvantageous in that making mechanical adjustments for an occupant other than the vehicle's driver is tedious and time-consuming because such an occupant cannot usually be specified and new mechanical adjustments need to be made each time they change.
Japanese laid-open patent publication No. 7-186880 reveals a system for controlling an occupant restraint apparatus by measuring the weight of an occupant with a weight sensor and a tilt sensor. The revealed system is, however, constructed of a large number of parts and considerably expensive due to a complex corrective logic because the difference between the detected weight of the occupant and the actual weight of the occupant is compensated for in view of the manner in which the occupant is seated on the seat cushion and the angle of the seatback.
Another problem is that increased restraint forces are applied to an occupant when an air bag is inflated. More specifically, as shown in FIG. 21 of the accompanying drawings, even if an acceleration .alpha. of the occupant caused with respect to the ground surface (corresponding to an impact force on the occupant) upon a collision of the vehicle is to be kept constant at a time t0 by paying out the seat belt under the constant EA load, when the air bag is inflated at a time t1, the acceleration .alpha. of the occupant with respect to the ground surface tends to increase due to restraint forces of the air bag.
There is known a process of adjusting the length by which the seat belt is paid out to-reduce the EA load when the EA mechanism is operated, as disclosed in Japanese laid-open patent publications Nos. 8-127313 and 8-268224, for example (see FIG. 22 of the accompanying drawings). According to the disclosed process, the paid-out length of the seat belt for reducing the EA load is set to a predetermined value L'1 which is uniquely fixed and cannot be varied.
The position of an occupant seated on the seat cushion in the longitudinal direction of the vehicle, i.e., the position of the seat slide, varies from body shape to body shape. As a result, as indicated at in FIG. 22, the relative inflating timing of the air bag 1 thorough 3 is varied. As shown in FIG. 23 of the accompanying drawings, the acceleration .alpha. of the occupant with respect to the ground surface varies depending on the position of the seat slide, resulting in an unstable occupant restraining capability.
In the EA mechanism, as shown in FIG. 24 of the accompanying drawings, even if a retractor 2 keeps the webbing tension (EA load) Ts at a constant level, as an occupant 3 moves forward, a belt restraint force Fs acting on the occupant 3 increases. The belt restraint force Fs acting on the occupant 3 is related to the webbing tension Ts by Fs=Ts cos.theta.s. As the occupant 3 moves forward from the solid-line position to the two-dot-and-dash-line position in FIG. 24, the angle .theta.s at which the webbing tension Ts acts becomes acuter, increasing the belt restraint force Fs (see FIG. 25 of the accompanying drawings). FIG. 25 shows a cross section along the seat belt over the occupant 3 shown in FIG. 4.
Therefore, as shown in FIG. 26 of the accompanying drawings, even if the webbing tension Ts is kept at a constant level by the retractor 2, the belt restraint force Fs acting on the occupant 3 increases as the occupant 3 moves forward.
Therefore, as indicated by the equation Fs=m.multidot..alpha. (m: mass of the occupant 3 and .alpha.: acceleration or deceleration on the chest of the occupant 3), the acceleration or deceleration .alpha. on the chest of the occupant 3, i.e., the impact force increases in proportion to the belt restraint force Fs as the occupant 3 moves forward (see FIG. 27 of the accompanying drawings).
FIGS. 28 and 29 of the accompanying drawings show the belt restraint force Fs acting on an occupant, represented by the vertical axis as it is related to the displacement x' of the occupant, represented by the horizontal axis, according to Fs=m.multidot..alpha., for the purpose of considering energy absorbing efficiency.
Even if the webbing tension Ts is kept constant by the retractor 2, since the belt restraint force Fs is not constant due to variations in the webbing tension angle .theta.s, when the EA mechanism is in its initial stage of operation, a dead zone where the EA mechanism does not work due to the difference between the maximum restraint forces and the restraint forces during movement of the occupant 3 is created as shown hatched in FIG. 28 of the-accompanying drawings, resulting in a poor efficiency for absorbing the impact energy.
Theoretically, insofar as the belt restraint force Fs is constant, the impact energy can effectively be absorbed from the initial stage of operation of the EA mechanism, making it possible to reduce the maximum restraint forces and the maximum deceleration, i.e., the impact force, as indicated by the dot-and-dash line in FIG. 29.
However, when the belt restraint force Fs acting on the occupant 3 is constant, the webbing tension Ts needs to be reduced depending on the paid-out length of the webbing during operation of the EA mechanism, in view of changes in the webbing tension angle .theta.s upon forward movement of the occupant 3, as indicated by the dot-and-dash line in FIG. 26.